...
What
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do
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numerical
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instability
...
warnings
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indicate?
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Answer:
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Numerical
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instability
...
warnings
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indicate
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that
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digits
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of
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accuracy
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were
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lost
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during
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computation.
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Software
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performs
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computation
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with
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16
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digits
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of
...
accuracy,
...
some
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of
...
which
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may
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be
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lost
...
when
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analysis
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involves
...
systems
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of
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significantly
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different
...
stiffness,
...
in
...
which
...
lower-order
...
values
...
are
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truncated
...
to
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suit
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operations
...
with
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higher-order
...
values.
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When
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this
...
warning
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occurs,
...
...
reports
...
the
...
location
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of
...
the
...
occurrence
...
such
...
that
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the
...
user
...
may
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edit
...
the
...
model
...
if
...
necessary.
...
Some
...
situations
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where
...
numerical
...
instability
...
may
...
occur
...
include:
...
- When the rigid-diaphragm assumption is applied to a slab with flexible interconnecting objects.
- When multiple releases are assigned to a joint, causing joint DOF to become orphaned (without stiffness). In this instance, numerical instability warnings indicate a modeling problem which should be corrected.
Depending on the number of digits lost, actions which result include the following:
- Less than 6 does not generate a response in that values and solution are sufficiently accurate.
- Between 6 and 11 lost digits of accuracy may or may not create numerical problems, therefore warning message is reported which reads: The results of the analysis may still be acceptable but the user should carefully check the results, especially the global sums of loads.
- Over 11 digits lost causes analysis to terminate because results may not be sufficiently accurate. ETABS indicates the location of the numerical problem, and presents a warning message which prompts the user to check and revise the model.